Sensor arrangements of this type are already employed in different embodiments, in motor vehicles, for example. For example, with “Hall elements” as rotational speed sensors on the wheels for an antilock braking system (ABS), as rotational speed and phase sensors for engine management, or as steering-angle sensors for electronic stability systems, and for electronic power steering systems. These sensor arrangements, which are known from DE 197 50 304 A1, for example, typically emit digital signals, e.g., switching flanks, as a function of a trigger wheel rotating in front of the sensor.
Due to mechanical tolerances in particular, the most important requirements on these rotational speed sensors in an ABS and in engine and transmission systems include the greatest possible air gap and a high immunity to vibrations. A number of, at times, contradictory requirements are also placed on these sensor arrangements, whereby a highly sensitive sensor is also highly sensitive to excitation by the vibrations that interfere with the measured result. Full functionality should be achieved for very large air gaps, i.e., a highly sensitive sensor. At the same time, when air gaps are small, the aim should be to prevent the occurrence of faulty signals caused by vibrations when a sensor signal is high.
To minimize the sensitivity to vibrations of sensor arrangements of this type, a variable hysteresis is often used in conventional rotational speed sensors. In this case, the signal amplitudes must first be measured and the hysteresis is then adapted in a flexible manner. A large hysteresis is used for high input signals, and a correspondingly reduced hysteresis is selected for small input signals, i.e., the amplitude required for switching is increased when the air gap is small.
At the same time, the sensor is also required to be immune to vibrations, particularly when the trigger wheel is at a standstill; this works against high sensitivity and makes it necessary to realize a large switching hysteresis.
A further aim is for the realized sensor to be insensitive to short-term signal changes, in particular to a marked amplitude reduction during operation. A main disadvantage of this method with a flexible hysteresis is therefore the loss of immunity against air gap impacts during operation in particular, which can generate a considerable, short-term reduction of the signal amplitude of this very type. As a result of a previously increased hysteresis at the switching point of the sensor, an air gap impact of this type may result in a loss of signal and/or a signal breakdown.
Furthermore, the sensor need only be calibrated first for the method to be used with an adaptable hysteresis, since the signal amplitude is not known until after calibration. To correctly adjust the hysteresis, the sensor would first have to measure the signal amplitude, however. Since no measured values are available immediately after switching on, a starting value—typically the minimum—for the hysteresis must be selected in the sensor. At the same time, this also means that the sensor is very sensitive to vibrations in this state, however.
Moreover, due to an increased hysteresis resulting from magnetic stimulation (i.e., during rotation of the trigger wheel), the sensor also loses its robustness to air gap impacts, which can drastically reduce the signal amplitude over a few periods.
For example, the use of an adaptive hysteresis that depends on the signal amplitude is known from U.S. Pat. No. 5,451,891 A1. In this case, a coupling factor is determined as the quotient of the measured sensor amplitude and the frequency and, based on this coupling factor, the hysteresis is adjusted in proportion to the product of the coupling factor and the frequency. With this known method, it is possible only to compensate for the behavior of passive sensors which deliver a very small signal for low excitation frequencies and output a very high amplitude for high frequencies. The behavior of sensors that deliver a constant internal signal amplitude independent of the signal frequency cannot be improved, however.